The overall objective of the proposed research is to define, at the level of chromatin regulation, a novel nuclear role of Wiskott-Aldrich syndrome protein (WASp) in the molecular and transcriptional pathways that program CD4 T cell (Th)1 differentiation and the development of type 1-pathogen-specific immunity. TBX21 gene encodes T-BET protein, the Th1-master regulator, which in concert with RUNX3 drives the activation of the target cytokine gene, IFNG. These activation events are important for specifying the Th1 cell fate. T cells from children with Wiskott-Aldrich syndrome (WAS), which lack expression of WASp, exhibit a concomitant deficiency of T-BET, fail to produce Th1-cytokine IFN, and develop severe systemic infections from intracellular pathogens, indicating a collective failure to develop a protective Th1-immune response. In contrast, there are indications that the Th2-response in a subset of WAS patients and in some murine models of WAS is pathologically exaggerated. Accordingly, the development of autoimmune colitis in some murine WAS model is Th2-cytokine-driven. Collectively, these observations suggests that an unbalanced activation of Th1 versus Th2 immunity may underlie the pathophysiology of WAS and its attendant complications. The specific objective for this application, therefore, is to understand WASp's role in the regulation of the Th1 network genes at a chromatin level, and clarify the molecular underpinnings of Th1-specific immune deficiency in human WAS. Using a combination of different biochemical approaches, we will delineate the protein:protein interactions of WASp in the Th1 cell genome (Aim1), test the role of WASp in local epigenetic modifications that favor productive transcription, and then identify perturbations in these regulatory events that may result from the absence of WASp (Aim 2) or from the presence of mutated WASp (Aim 3). These studies are expected to elucidate a previously unexplored, novel function of WASp in the epigenetic and transcriptional control of its target immune function genes in humans, and unearth an entirely new molecular basis for systemic immunodeficiency and the associated complications in the human WAS.